Tilting cam, rotating barrel pumps are known per se. Such pumps comprise a rotatable barrel containing pumping pistons, a tiltable pump cam operatively connected to the pumping pistons, and a proportional servo actuator operatively connected to the pump cam. The pump cam is connected to the barrel such that, when the pump cam is perpendicular to the axis of the barrel, the pumping pistons are not displaced axially and no fluid is pumped, and the more the pump cam is tilted relative to the axis of the barrel, the more the pumping pistons are displaced axially and the more fluid is pumped. The proportional servo actuator includes an output member which bears against the pump cam in a direction parallel to the axis of the barrel. As the output member is translated backward and forward, the pump cam is caused to tilt relative to the barrel.